This invention relates to solar panels adapted for utilising the heat produced by solar radiation.
In known solar panels, a flat plate blackened on the front to improve absorption of solar radiation is arranged with its blackened surface facing the sun and sloped at a suitable angle to optimize the energy collected. A series of tubes are secured to the panel, and water to be heated is circulated through these tubes to extract the heat received by the panel. The back of the panel is often insulated by a layer of insulating material such as glass wool or plastic foam.
In most cases these panels include a frontal screen made of glass or transparent plastics material such as transparent acrylic or PVC, which allows solar radiation to pass through the screen onto the panel and retains the heat by reducing losses caused by re-radiation or convection cooling.
The warmed water from solar panels is normally circulated through a separate tank, so that the temperature may build up to a maximum value being a balance between the heat input and heat losses in the system. This water is then used as feed water for the main hot water tank, as hot water is drawn off from the system. As this water is generally only warm, except in very hot weather or when the panel is used in hot climates, it cannot be used directly as hot water, and a separate heater in the main hot water tank is necessary to heat the water to usable temperatures. It is also clear that water from the solar panel cannot be used generally for maintaining the temperature of the hot water in the main tank.
Some larger installations utilise heat exchangers of varying complexity. The heated liquid circulating through the panel is passed in tubes through a tank, so that the heat is transferred into the water in this tank. This water may be used indirectly as described above or further "heat pumps" may be used to extract the heat from the water so that it can be used for heating water in the main hot water system of the installation.
A number of factors limit the efficiency of known panels. They are relatively useful for heating water in hot climates, but in general they only result in warm water in temperate climates, suitably only for swimming pools except in very hot weather. The flat black plate heated by the solar radiation also heats the air trapped between the solar plate and the front window, and some of this heat is lost by air convection and conduction through this window. Some heat is also lost through the back layer of insulating material.
Solar panels also suffer from cold and icy conditions, where the temperature is such that circulating water in the panel may freeze and burst the pipes. This can be overcome by draining the water from the panel in cold weather, but if an anti-freeze agent is used to prevent this occurring, then a heat exchanger must be used to extract the heat from this liquid.
As solar heat only falls during part of the day, and not at all at night, the panel is heated only at irregular intervals. Liquid circulating through the solar panel will gain heat during these intervals, but will lose heat at other times when the panel acts as a radiator. This is usually overcome in known panels by using a thermostatically operated switch, which controls the circulation so that it only occurs when the temperature exceeds a pre-set level, or is higher than the temperature of water in the storage tank.
In most known panels, water to be heated is passed into the panels at the bottom, travels up through the panels in the tubes and out at the top of the panel. In order to interconnect adjacent panels to increase the heating effect, the inlet pipes and the outlet pipes are connected respectively, but to ensure uniform heating effects and to maintain high panel efficiency, careful balancing of the water flow is necessary.
Other solar installations have been built, and various methods proposed for focusing the solar rays onto collectors in order to concentrate the energy received. In this way, higher temperatures are achieved and losses reduced, but only at the expense of complication and cost. The parabolic reflectors or focusing devices used must be steered to follow the direction of the sun, which requires expensive and complicated equipment.
It is an object of the present invention to eliminate or minimise the disadvantages of known solar panels by providing a method and apparatus for solar heating which is capable of providing water hot enough to feed straight into a main hot water tank, thus eliminating the need for a separate warm water tank. It is a further object of the invention to utilise a refluxing condenser system incorporating a liquid which will not freeze even in the coldest weather, so that the problem of bursting pipes is overcome. A further object of the invention is to reduce heat losses from the hot water during both hot and cold periods to very low values, and to provide a design of panel in which the interconnection of adjacent panels is simplified and the requirement for flow equilization controls is eliminated. Furring up of the narrow channels in the panel will not occur, and corrosion of these channels is eliminated.
A still further object of the invention is to provide means for concentrating the heat without the use of optical focusing.